Microestructuras en el granito de Aguas Blancas, Sierras Australes de Buenos Aires.

LUIS DIMIERI; LAURA GRECCO,; MARÍA FRISICALE,

Revista de La Asociación Argentina de Mineralogía Petrología y Sedimentología

Asociación Argentina de Mineralogía Petrología y Sedimentología

Lugar: Buenos Aires; Año: 1990 vol. 21 p. 53 - 60

0325-0253

The analysis of the granitic bodies that outcrop at Aguas Blancas, near Dufaur locality, Buenos Aires province, was carried on in order to study the deformation which affected them. This work is accomplished by means of microscopical analysis and the objetive is the identification of the microstructures and the interpretation of the environmental deformation conditions. The granitic rocks of 574 m.y. old are overlain by a paleozoic sedimentary cover; petrographycally they are pegmatitic, aplitic and granophyric granites. Two main structural domains are recognized; one is cha'racterized by zones not affected by mesoscopic penetrative foliation; the other is represented by very intense foliated narrow zones. The former domain is considered in this work. The microstructures observed in quartz, feldspar and mica crystals are widespread ondulose extinction, deformation bands notably in feldspar, very incipient deformation lamellae, very common Bohm lamellae, abundant feldspar deformation twins and considerable microfracturation. Subgrain development is important and strain-free neoformation quartz grains are observed. There are few examples of the presence of thin, dark planar discontinuities associated with considerable amounts of oriented layer-silicates minerals. Related with those discontinuities, some evidence of shear movements (S-C structures) is present together with few pressure shadows features around feldspar crystals. Recovery and primary recrystallization processes are responsible for the microstructural changes that took place in the deformed crystals, namely, subgrain, poligonization and strains·free grain development, Both processes allow a steady state flow which is a necessary condition for the attainment of large strains in rocks. Dislocation movement along slip systems by translation gliding and twinning involving climb and cross-slip is the most predominant deformation mechanism likely to explain the studied microstructures. Also some evidence is observed supporting grain boundary diffusion (Coble creep processes) with pressure solution being a particular case. The environment of deformation inferred from the microstructural evidence corresponds to conditions of confining stress and temperature equivalent to a transition between low grade metamorphism and low green-schist facies.